The configuration of a conventional ultrasonic transducer is shown in FIG. 6. Most conventional ultrasonic transducers are resonant ultrasonic transducers using a piezoelectric ceramic as a vibrating element. The ultrasonic transducer shown in FIG. 6 uses the piezoelectric ceramic as the vibrating element to perform both conversion from an electric signal to ultrasonic waves and conversion from ultrasonic waves to the electric signal (transmission and reception of ultrasonic waves). The bimorph-type ultrasonic transducer shown in FIG. 6 comprises two piezoelectric ceramics 61 and 62, a cone 63, a case 64, leads 65 and 66, and a screen 67.
The piezoelectric ceramics 61 and 62 are stuck together, and the leads 65 and 66 are respectively connected to the ceramics 61 and 62 at the surfaces thereof opposite to the stuck surface.
Since the resonant ultrasonic transducer uses a resonance phenomena of the piezoelectric ceramic, excellent ultrasonic transmission and reception characteristics can be obtained only in a relatively narrow frequency band near the resonance frequency. As shown by the curve Q2 in FIG. 8, the frequency characteristic of the resonant ultrasonic transducer is −30 dB with respect to the maximum sound pressure for a frequency of ±5 kHz with respect to a center frequency (resonance frequency of the piezoelectric ceramic) having a maximum sound pressure of for example 40 kHz.
In addition to the resonant ultrasonic transducer shown in FIG. 6, the electrostatic ultrasonic transducer has been heretofore known as a broadband oscillation-type ultrasonic transducer as disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-50392, which can generate relatively high sound pressure over a wide frequency band. The electrostatic ultrasonic transducer is referred to as a Pull type, since a vibrating film works only in a direction attracted to a fixed electrode side.
FIG. 7 shows a specific configuration of the broadband oscillation-type ultrasonic transducer (Pull type).
The electrostatic ultrasonic transducer shown in FIG. 7 uses a dielectric film 131 (insulator) such as a PET (polyethylene terephthalate resin) having a thickness of about 3 to 10 μm, as a vibrating film.
An upper electrode 132 formed as a metal foil of aluminum or the like, is integrally formed with the dielectric 131 on the upper face thereof by a process such as vacuum evaporation, and a lower electrode 133 formed of brass is provided so as to come in contact with the lower face of the dielectric 131. The lower electrode 133 is connected with a lead 152, and is fixed to a base plate 135 formed of bakelite or the like.
The upper electrode 132 is connected with a lead 153, which in turn is connected to a DC bias power supply 150. A DC bias voltage for attracting the upper electrode, of about 50 to 150 V is applied to the upper electrode 132 at all times by the DC bias power supply 150, so that the upper electrode 132 is attracted to the lower electrode 133 side. A signal source 151 is connected to the lower electrode 133.
The dielectric 131, the upper electrode 132, and the base plate 135 are tightly fitted in the case 130 together with metal rings 136, 137 and 138, and a mesh 139.
A plurality of fine grooves of about several tens to several hundred μm having a irregular, nonuniform shape is formed in the surface of the lower electrode 133 on the dielectric 131 side. The fine grooves form a gap between the lower electrode 133 and the dielectric 131, and hence, the distribution of capacitance between the upper electrode 132 and the lower electrode 133 slightly changes.
The random fine grooves are formed by roughening the surface of the lower electrode 133 manually with a rasp. The electrostatic ultrasonic transducer is thus formed with innumerable capacitors having different sizes of the gap and different depths in this manner. A rectangular wave signal (50 to 150 Vp-p) is applied between the upper electrode 132 and the lower electrode 133, with the DC bias voltage being applied to the upper electrode 132.
In the ultrasonic transducer having the above configuration, the frequency characteristic of the ultrasonic transducer shown in FIG. 7 becomes broadband as shown by a curve Q1 in FIG. 8. That is, the frequency characteristic of the electrostatic, broadband oscillation-type ultrasonic transducer is flat from 40 kHz to about 100 kHz, and at 100 kHz is about −6 dB as compared to the maximum sound pressure.
However, as shown in FIG. 8, regarding the maximum value of the sound pressure, the electrostatic ultrasonic transducer has a value as low as 120 dB or lower, as compared to 130 dB or higher for the resonant ultrasonic transducer. Hence the sound pressure is slightly insufficient for using it as an ultrasonic speaker.
Here, explanation will be given of the ultrasonic speaker in which the ultrasonic transducer is utilized. In the ultrasonic speaker, a signal in an ultrasonic frequency band referred to as a carrier wave, is AM modulated by an audio signal (a signal in an audio-frequency band), and the ultrasonic transducer is driven by the modulated signal. Thereby, sound waves in a state with ultrasonic waves being modulated by an audio signal from a signal source are radiated to the air, so that the original audio signal is self-reproduced in the air due to the nonlinearity of the air.
More specifically, since the sound waves are compression waves that propagate through the air as a medium, dense parts and sparse parts of the air appear remarkably in a process of propagation of the modulated ultrasonic waves. Since the speed of sound is fast in the dense parts and is slow in the sparse parts, a distortion occurs in the modulated wave itself. As a result, the waveform is separated into carrier waves (ultrasonic wave) and audio waves (original audio signal), and a human can hear only the audio sound (original audio signal) of 20 kHz or below. This principle is generally referred to as a parametric array effect.
An ultrasonic sound pressure of not lower than 120 dB is necessary in order that the parametric array effect appears sufficiently, but it is difficult to achieve this figure by the electrostatic ultrasonic transducer. Hence, a ceramic piezoelectric element such as PZT or a polymer piezoelectric element such as PVDF has been used as an ultrasonic wave-transmitting member.
However, the piezoelectric element has a sharp resonance point regardless of the material, and is driven at the resonance frequency and put to practical use as an ultrasonic speaker. Therefore, the frequency domain that can ensure a high sound pressure is quite narrow. That is, it can be said that the piezoelectric element has eventually a narrow-band.
Generally, the maximum audio frequency band of a human being is about 20 Hz to 20 kHz, with a band of about 20 kHz. That is, in the ultrasonic speaker, the original audio signal cannot be demodulated with fidelity, unless a high sound pressure is ensured over the frequency band of 20 kHz in the ultrasonic region.
It can be easily understood that it is difficult to reproduce (demodulate) the broadband of 20 kHz with fidelity with the resonant ultrasonic speaker using the conventional piezoelectric element.
Actually, the ultrasonic speaker using the conventional resonant ultrasonic transducer shown in FIG. 6 has the following problems: (1) the band is narrow and reproduced sound quality is low; (2) if the AM modulation factor is too high, the demodulated sound is distorted, and hence the modulation factor can be increased up to about 0.5 at maximum; (3) if the input voltage is increased (if the volume is increased), vibration of the piezoelectric element becomes unstable, and the sound is distorted When the voltage is further increased, the piezoelectric element itself is likely to be broken; and (4) arraying, enlargement, and miniaturization are difficult, and hence the production cost is high.
On the other hand, as is disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-50387, the ultrasonic speaker using the electrostatic ultrasonic transducer (Pull type) shown in FIG. 7 can substantially solve the problems of the aforementioned conventional technology, and can cover a wide band. However there is a problem in that the absolute sound pressure is not sufficient for the demodulated sound to have sufficient volume.
Further, in the Pull-type ultrasonic transducer, the electrostatic force works only in a direction attracting toward the fixed electrode side, and the symmetry property of vibration of the vibrating film (corresponding to the upper electrode 132 in FIG. 7) cannot be maintained. Therefore, there is a problem in that when the Pull-type ultrasonic transducer is used for the ultrasonic speaker, vibration of the vibrating film directly generates audible sound.